JPH0526709A - Coriolis mass flowmeter - Google Patents

Abstract

PURPOSE:To reduce span fluctuation due to a change in temperature by a construction wherein the ratio in natural frequency between a drive mode and a vibration mode is made to be a specified value or below so that a span error occurring in a measuring tube be within a prescribed value. CONSTITUTION:A construction is so made that the ratio between the natural frequency in a drive mode and the natural frequency in a vibration mode is a prescribed value or below so that a span error occurring on the basis of a damping coefficient according to the material of a measuring tube 1 be within a prescribed value. In order to make the span error be 1% or below in the case when the tube 1 is a metal tube and a damping ratio is 0.01, for instance, a spring constant of a spring 6 is set so that the ratio in the natural frequency be 0.9. When the ratio in the natural frequency is made too small, the same condition without the spring 6 is brought forth and a measuring signal can not be made large. When the tube 1 is an ordinary metal tube, accordingly, it is desirable that the ratio in the natural frequency is 0.7 to 0.95, and it is desirable that the ratio is 0.5 to 0.9 when the tube is a plastic tube. As the result, span fluctuation can be reduced even when the damping ratio of the tube 1 changes due to a change in temperature or others.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Coriolis mass flowmeter having excellent characteristics such that the span variation is small even if the damping ratio of the measuring tube changes due to temperature changes or the like.

[0002]

2. Description of the Related Art FIG. 4 is an explanatory view of the configuration of a conventional example which has been generally used, for example, Japanese Patent Laid-Open No. 58-11741.
No. 6, title of invention: “Flowmeter”, patent applicant: applicant of the present application. In the figure, 1 is a U-shaped measuring tube having both ends attached to the piping A. Reference numeral 2 is a mounting base for the measuring tube 1. Reference numeral 3 is a vibrator provided at the tip of the U-shaped measuring tube 1. Reference numerals 4 and 5 are displacement detection sensors provided on both sides of the measuring tube 1, respectively. 6 is a spring, one end of which is a Coriolis vibration mode (vibration mode expressed by Coriolis force, in this case, torsional vibration (asymmetrical flexural vibration)), which is located at the center of the measuring tube 1. They are connected and the other end is fixed to the base 2.

In the above structure, the measuring fluid is flown through the measuring tube 1 to drive the vibrator 3. Assuming that the angular velocity “ω” in the vibration direction of the vibrator 3 and the flow velocity “V” of the measured fluid (hereinafter, the symbol enclosed in “” represents a vector amount), Fc = −2 m “ω” × “V” The mass flow rate can be measured by measuring the amplitude of vibration that is proportional to the Coriolis force acting on the Coriolis force.

However, in general, the amplitude of vibration proportional to the Coriolis force is much smaller than the amplitude of vibration due to excitation, and the amplitude of vibration proportional to the Coriolis force cannot be directly detected.

Now, when viewed from the direction of Z in FIG. 4, considering the vibration directions of α and β by vibrating the vibrator 3,
As shown in FIG. 5 and FIG. 6, the direction of the Coriolis force differs depending on the direction of the flow velocity “V”, so that the phases are reversed and the measuring tube 1 vibrates while twisting. This is detected by the displacement detection sensors 4, 5, for example, magnetic sensors, and the displacement detection sensors 4, 5 are detected.
Since the phase difference of the displacement of is proportional to (amplitude of vibration proportional to Coriolis force) / (amplitude of vibration due to excitation), the mass flow rate can be obtained. Since the phase difference can be measured as the time difference Δt when the waveforms cross zero, the Coriolis force can be measured as a result.

When a portion serving as a node of Coriolis vibration mode (in this case, torsional vibration (asymmetrical flexural vibration)) is restrained by the spring 6, a driving mode of the measuring tube 1 (represented by a driving means). In vibration mode, in this case, the natural frequency of longitudinal vibration (symmetrical flexural vibration) rises, while in Coriolis vibration mode (in this case, torsional vibration (asymmetrical flexural vibration)) It has almost no effect on the natural frequency. Further, generally, the natural frequency of the torsional vibration (asymmetrical flexural vibration) of the measuring tube 1 is higher than the natural frequency of the longitudinal vibration (symmetrical flexural vibration).

Therefore, by using the spring 6 having an appropriate constant, the natural frequency of the driving mode of the measuring tube 1 (in this case, the longitudinal vibration (symmetrical bending vibration)) is increased to increase the Coriolis vibration mode. (In this case, torsional vibration (asymmetric bending vibration)) can be made substantially equal to the natural frequency.

With this structure, the displacement of the measuring tube 1 caused by the Coriolis force is amplified by the Q of Coriolis vibration mode (in this case, torsional vibration (asymmetric bending vibration)). Therefore, the detection sensitivity can be greatly improved.

[0009]

However, in such a device, when the natural frequency of the drive mode and the natural frequency of the Coriolis vibration mode are substantially matched, the amplitude of the Coriolis vibration is It becomes dependent on the magnitude Q of resonance. When Q changes with temperature or the like, the magnitude of the signal proportional to the flow rate also changes at the same time, causing span fluctuation.

The present invention solves this problem. It is an object of the present invention to provide a Coriolis mass flowmeter which has a good characteristic that the span variation is small even if the attenuation ratio of the measuring tube changes due to temperature change or the like.

[0011]

In order to achieve this object, the present invention allows a measuring fluid to flow in an oscillating measuring tube, and the Coriolis force generated by the flow and the angular vibration of the measuring tube causes the measuring tube to deform and vibrate. In the Coriolis mass flowmeter, the nodes of the Coriolis vibration mode of the measuring tube are constrained via elastic materials, and the natural frequency of the drive mode and the natural vibration of the Coriolis vibration mode are constrained. In a Coriolis mass flowmeter whose number is substantially the same, the natural frequency of the drive mode and the vibration are adjusted so that the span error generated based on the damping coefficient of the material of the measuring tube is within a predetermined value. This is a Coriolis mass flowmeter characterized in that the ratio with the natural frequency of the mode is set to a predetermined value or less.

[0012]

In the above structure, the natural frequency of the drive mode and the vibration mode are set so that the span error generated based on the damping coefficient of the material of the measuring tube is within a predetermined value.
Since the ratio with respect to the natural frequency of the coil is configured to be equal to or less than the predetermined value, a Coriolis mass flowmeter with a small span variation can be obtained even if the damping ratio of the measuring pipe changes due to temperature change or the like. Hereinafter, detailed description will be given based on examples.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an explanatory view of the essential structure of an embodiment of the present invention. In the figure, 1 is a U-shaped measuring tube having both ends attached to the piping A. Reference numeral 2 is a mounting base for the measuring tube 1. Reference numeral 3 is a vibrator provided at the tip of the U-shaped measuring tube 1. Reference numerals 4 and 5 are displacement detection sensors provided on both sides of the measuring tube 1, respectively. 6 is a spring, one end of which is a node of Coriolis vibration mode (vibration mode expressed by Coriolis force, in this case, torsional vibration (asymmetric flexural vibration)) at the center of the measuring tube 1. They are connected and the other end is fixed to the base 2.

Thus, the natural frequency of the drive mode and the natural frequency of the vibration mode are set so that the span error generated based on the damping coefficient of the material of the measuring tube 1 is within a predetermined value. Is configured to be equal to or less than a predetermined value. In this case, the spring constant of the spring 6 is determined so that the ratio of the natural frequency of the drive mode and the natural frequency of the vibration mode is 0.9. However, when the measuring tube 1 is a normal metal tube, the natural frequency ratio is preferably 0.7 to 0.95, and the plastic tube is preferably 0.5 to 0.9.

That is, the damping ratio of the measuring tube 1 changes due to temperature changes and the like. FIG. 2 shows the span error when the damping ratio changes by 50% for some damping ratios. As can be seen from FIG. 2, in order to keep the span error below a certain value, the natural frequency ratio must be below a certain value. For example, when the measuring pipe 1 is a metal pipe and the damping ratio is 0.01, the natural frequency ratio may be set to 0.9 or less to reduce the span error to 1% or less. Generally, since the damping ratio of metal is 0.01 or less, the span error can be sufficiently reduced by setting the natural frequency ratio to 0.95 or less. When the measuring pipe 1 is a plastic pipe, the damping ratio is larger than that of metal. For example, in the case of a plastic with a damping ratio of 0.03, the natural frequency ratio is 0.
It may be 77 or less. There are various materials for plastic, and the range of the damping ratio is large. Therefore, the natural frequency ratio may be 0.9 or less.

If the natural frequency ratio is made too small, it becomes the same as the case without the spring 6, and the measurement signal cannot be increased. Therefore, when the measuring pipe 1 is a normal metal pipe, the natural frequency ratio is preferably 0.7 to 0.95, and the plastic pipe is preferably 0.5 to 0.9.

In the above structure, the natural frequency of the drive mode and the natural frequency of the vibration mode are set so that the span error generated based on the damping coefficient of the material of the measuring tube is within a predetermined value. Since it has been configured such that the ratio of and is less than or equal to a predetermined value, a Coriolis mass flowmeter with less span fluctuation can be obtained even if the damping ratio of the measuring pipe changes due to temperature changes or the like.

As a result, it is possible to obtain a Coriolis mass flowmeter which has a small span variation and excellent characteristics.

FIG. 3 is an explanatory view of the essential structure of another embodiment of the present invention. In this embodiment, the measuring pipe 7 is a straight pipe.

In the above-mentioned embodiment, the example of the single pipe has been described, but the measuring pipe 1 may be composed of a parallel bent pipe or a straight pipe branched into two. In that case, the spring 6 is coupled between the two measuring tubes 1. Also, the spring 6
May be a spring other than a simple spring or a leaf spring.

[0021]

As described above, the present invention is a Coriolis mass flowmeter in which a measuring fluid is caused to flow in an oscillating measuring tube, and the flow and the Coriolis force generated by the angular vibration of the measuring tube cause the measuring tube to deform and vibrate. Therefore, the node of the Coriolis vibration mode of the measuring tube is constrained via an elastic material so that the natural frequency of the drive mode and the natural frequency of the Coriolis vibration mode are substantially equal. In the Coriolis mass flowmeter, the natural frequency of the drive mode and the natural vibration of the vibration mode are adjusted so that the span error generated based on the damping coefficient of the material of the measuring tube is within a predetermined value. A Coriolis mass flowmeter characterized in that the ratio to the number is set to a predetermined value or less.

As a result, it is possible to obtain a Coriolis mass flowmeter which has a small span variation and excellent characteristics.

Therefore, according to the present invention, it is possible to realize a Coriolis mass flowmeter having a good characteristic that the span variation is small even if the damping ratio of the measuring pipe changes due to temperature change or the like.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a main part configuration of an embodiment of the present invention.

FIG. 2 is an operation explanatory diagram of FIG.

FIG. 3 is an explanatory diagram of a main part configuration of another embodiment of the present invention.

FIG. 4 is an explanatory diagram of a configuration of a conventional example that is generally used in the past.

5 is an operation explanatory diagram of FIG. 1. FIG.

FIG. 6 is an operation explanatory diagram of FIG. 1;

[Explanation of symbols]

 1 ... Measuring tube 2 ... Case 3 ... Transducer 4 ... Displacement detecting sensor 5 ... Displacement detecting sensor 6 ... Spring 7 ... Measuring tube

Claims (1)

Claim: What is claimed is: 1. A Coriolis mass flowmeter for causing a measuring fluid to flow in an oscillating measuring tube, and deforming and vibrating the measuring tube by a Coriolis force generated by the flow and the angular vibration of the measuring tube. Coriolis that constrains the nodes of the Coriolis vibration mode of the measuring tube via an elastic material so that the natural frequency of the drive mode and the natural frequency of the Coriolis vibration mode are approximately the same. In the mass flow meter, the ratio of the natural frequency of the drive mode and the natural frequency of the vibration mode so that the span error generated based on the damping coefficient of the material of the measuring tube is within a predetermined value. A Coriolis mass flowmeter characterized in that is set to be a predetermined value or less.